Accidentally skipped through the class descriptions, so I figured I'd make a list of all the class perks and see which one works best. Not sure if this is going to be at all helpful, but it's a place to start. Feel free to contribute with what you find. I noticed that this was a beta, so these may change in the final game?

Marksman IAmbushDescription: When a member of the Rear Guard joins the Front Line to perform an attack, they will have a small chance to fully de-sync their target.Mastery: 100Prereqs: noneType: Rear Guard, Passive

Marksman IVAt the ReadyDescription: Small chance to perform a Ranged counter after being targeted by a Ranged attack. The counter does not consume AP.Mastery: 1500Prereqs: Master two Level 3 Mastery SkillsType: Front Line, Passive

Brute IThrowing AxeDescription: When equipped, there will be a chance to deal double damage with Knife ThrowMastery: 100Prereqs: noneType: Rear Guard, Passive

Brute IICoup de Grace(40AP)Description: Launch a vicious attack when the enemy Front Line has been synchronized. Can only be used when your guild has a lower score than the enemy guild. Mastery: 300Prereqs: Master one Level 1 Mastery SkillType: Front Line, Active